This invention relates to tachometers, and more particularly relates to tachometers for displaying the speed of an internal combustion engine.
A number of circuits have been devised for use as tachometers in connection with internal combustion engines, but each has exhibited limited accuracy and relatively high cost which has discouraged its use. For example, U.S. Pat. No. 3,219,926 (Dion-Nov. 23, 1965) describes a tachometer utilizing a Zener diode-inductor filter which is responsive to ignition coil primary voltage waveforms. The Dion filter produces a relatively rectangular pulse from each primary voltage waveform which is used to charge a capacitor 24. Capacitor 24 is discharged through a meter movement which indicates the speed of the engine. In order to operate at high speed, capacitor 24 must be rapidly discharged. If the discharge period is too long, the current pulse resulting from the trailing edge of one primary voltage waveform interferes with the current pulse resulting from the leading edge of the next primary voltage waveform, and the tachometer readings become inaccurate.
Two techniques can be used to reduce the capacitor discharge time. Firstly, the value of capacitor 24 can be lowered. However, this technique reduces the energy available to drive the Dion meter movement 27, and a more sensitive and costly meter must be employed. Secondly, the resistance of meter 27 can be decreased. This technique would increase the sensitivity of the meter, but also would increase its cost. Both of the foregoing techniques would result in a more expensive circuit.
The Dion circuit requires components with relatively close tolerances. For example, capacitor 24 must be held to a reasonably close tolerance and must have a relatively large value if an inexpensive and insensitive meter movement is used.
It has been found that the disadvantages of the prior art can be overcome by employing a pulse-forming means for removing at least some of the oscillations from the ignition pulses generated by an engine to form corresponding timing pulses. A meter capable of displaying the rate at which current pulses are received is also employed, although the meter may be an inexpensive low sensitivity type. A capacitor having first and second plates is connected in series with the meter, and a rectifier enables the transmission of a current pulse through the capacitor and meter in response to each timing pulse, so that the capacitor is charged to a predetermined voltage during each timing pulse. A discharge device momentarily interconnects the first and second plates of the capacitor in response to the termination of each timing pulse. When the first and second plates are momentarily interconnected, the voltage on the capacitor is dissipated without transmitting current through the meter. As a result, the meter receives current pulses flowing in only one direction so that the speed of the engine can be indicated depending on the frequency with which the current pulses are received by the meter.
The foregoing techniques offer a number of advantages over the prior art. A relatively insensitive, low-cost meter movement can be employed, and the charging capacitor can have a relatively low value and large tolerance. In addition, the use of this circuitry enables the rapid discharge of the capacitor, so that the tachometer can operate at high rates of engine speed.